Butterfly valves have historically been classified as low pressure valves that are typically utilized when the fluid pressure conditions to be encountered are of nominal nature. It is well known to be quite difficult to provide a butterfly valve mechanism having an annular sealing element that is capable of providing a positive seal about the periphery of a butterfly valve element in the closed position thereof, with positive sealing in both low and high pressure conditions. Butterfly valves have therefore been typically utilized in the past under conditions where a nominal amount of leakage is acceptable. Although butterfly valve leakage typically occurs because of internal sealing inadequacy, the trunnion sealing elements of many butterfly valve mechanisms also fail to provide an adequate seal and thereby cause a significant leakage problem.
Split valve bodies are incorporated into many different types of mechanisms, but in the case of butterfly valves, although split bodies are utilized to some degree, it is difficult to provide an efficiently designed split body assembly. Since the butterfly element must be supported by stem and trunnion openings of the valve body these openings will typically create stem and trunnion leakage problems that are difficult to overcome. It is therefore typical in the industry to provide integral valve body structures or, in the alternative, to provide valve body joints on a bias so that the stem and trunnion openings are not intersected. This of course adds materially to the cost of machining the butterfly valve body and therefore materially affects the market price of valves so manufactured.